1. Field of the Invention
This invention generally relates to a heating-fixing device. More particularly, it relates to a heating-fixing device having an improved heating member which is applicable in electrophotographic copying machines.
2. Description of the Prior Art
For copying machines, there are generally known some methods which include a method whereby an electrostatic latent image is formed on plain paper coated with a photoconductive material such as zinc oxide or the like and such image is directly developed, and a method whereby an electrostatic latent image is formed on a plate-like or a roller-like photosensitive medium formed of a photoconductive material such as selenium, CdS (cadmium sulfide) or the like and is developed into a visible image, whereafter the visible image is transferred to a separate transfer medium such as paper or the like. One of the important problems common to these electrophotographic copying methods is the heating-fixing step for fixing the developer (or toner) used for the image development.
For example, in a copying machine wherein a visible image formed on a photosensitive plate is transferred to a copying medium (e.g. paper or the like) to provide a transfer image, and if the transfer image consists of thermoplastic resin particles containing coloring pigments, the transfer image is plasticized and softened by being heated, or in the case of a developer (liquid toner) containing coloring pigments and thermoplastic resin in solvent, the particles are given adhesiveness by the solvent being gasified and dried, whereby the particles are coupled together and then solidified upon cooling of the copy medium, thus completing the fixation of the transfer image. Such copying process is completely automated within the copying machine so that the copy medium is conveyed through the machine, repeating meandering and curved motion. Of course, during the step of fixation also, the copy medium in complicatedly curved motion must be heated and fixed while being in such motion. Also, in order to effect heating-fixing most efficiently, it is usually essential that the copy medium be brought into intimate contact with the heat-emitting surface of the fixing device. For this reason, it is sometimes required that the heat-emitting surface of the fixing device be meandering and complicatedly curved. At the same time, the temperature distribution of the heat-emitting surface must be uniform in order to prevent irregular fixation of the copy image, and it is also necessary to provide a desired temperature distribution in accordance with the copy characteristics required. In this manner, a variety of functions are required for the heating-fixing device and in addition, the heating efficiency in the heating-fixing device is an important performance required for the heating-fixation. More specifically, the following requirements are involved:
1. In the heating-fixing device, the time required from after the closing of the main switch till the heating-fixing surface attains a prescribed temperature must be short (good in temperature rise);
2. The heating method must be good in thermal efficiency, because this is indispensable to reduction in power consumption and reduction in size of the copying machine; and
3. A better thermal efficiency must be provided which leads to a smaller size and accordingly, a smaller heat capacity, of the fixing device, which in turn reduces the heat transfer to the surrounding parts apainst their sharp temperature rise, so that the necessity of heat-resisting or adiabatic treatment is eliminated.
Heretofore, a heating member constructed as shown in FIG. 1 of the accompanying drawings has commonly been used in heating-fixing devices.
As shown there, a heat-resistant, insulative substrate 1 such as mica or the like carries thereon a heating element 2 forming a heating circuit of nichrome, the heating element 2 being coated with a heat-resistant, insulative layer 3 of mica or the like, and a plate of metal such as Al, Bs, Fe or the like is further overlaid as a good heat-conductive layer 4 over the layer 3, thereby forming a heating member. Copy medium P, such as photosensitive paper with its surface already developed or transfer medium with developing liquid already transferred thereto passes on the surface of the good heat-conductive layer 4 of the heating member while keeping contact therewith, whereby the developer is heated and fixed.
In such a conventional heating member, the temperature distribution on the surface thereof must be uniform throughout the entire surface area and accordingly, a material of good heat-conductivity, usually, metal, is desired and employed for such surface. However, metal is usually electrically conductive and must therefore be electrically insulated from the heating member, and this necessitates the provision of the heat-resistant, insulative layer 3. Usually, the electrically insulative material is not a good thermal conductor and it takes much time for the heat to reach the copy medium P from such material. Moreover, such material is very poor in thermal efficiency. In FIG. 1, reference numerals 5 and 5' designate rollers for transporting the copy medium P, and BL denotes a blast pressure for bringing the copy medium into intimate contact with the heating member.
Another known construction is the roller-shaped heating-fixing device as shown in FIG. 2. Fixation is accomplished by the copy medium P being held and passed between two heating rollers 6 and 6' rotating in the direction of arrow. Each of these heating rollers 6, 6' comprises a shaft 7, a heating element 8 and a heat-conductive cylinder 9, and a layer of air 10 having a high specific heat is provided between the heating element 8 and the heat-conductive cylinder 9 to electrically insulate them from each other. This has resulted in bad heat conduction, that is, the heat supplied from the heating element 8 to the heat-conductive cylinder 9 has been only the radiant heat, which has meant a very low thermal efficiency.